Al Gore and George W Bush said they had no choice but to go ahead with their third and final presidential debate in Missouri last night despite the death of the state governor, a key figure in Mr Gore's Democratic party, in a plane crash on Monday night.

Governor Mel Carnahan, who was running for election to the US Senate next month, was killed when a private plane taking him to a fundraising event crashed in poor weather about 30 miles south-west of St Louis, the scene of last night's debate.

The crash cast a pall over what was already a tense, high-stakes event in the presidential race. Both candidates agreed to pay tribute at the start of the debate to Carnahan and then go ahead with the debate as scheduled.

Carnahan, 66, died in a light-engine Cessna 335 piloted by his son Roger, 44, who also perished. Another passenger, Carnahan's campaign adviser, Chris Sifford, 37, was killed too.

Floral tributes were accumulating rapidly outside the governor's mansion in the state capital, Jefferson City, where Carnahan's widow Jean, his high school sweetheart, was being consoled by their three surviving children. Pres ident Clinton called Mrs Carnahan from Egypt to offer his condolences.

The death was big news across the US because Carnahan had been involved in a crucial part of the Democratic effort to retake the US Senate.

The popular and distinguished two-term governor was engaged in a neck-and-neck battle to unseat Senator John Ashcroft, a strong conservative, on November 7. Missouri is also a key swing state in the Gore-Bush race and one of five top target seats which the Democrats need to win to overturn the Republicans' current 54-46 majority in the Senate.

Mr Ashcroft suspended his campaign yesterday, and put all political advertising on hold "out of respect".

Under Missouri law, however, Carnahan's name must remain on the ballot because the deadline for nominations had closed, raising the macabre possibility that he could yet be elected posthumously. If that happens, and local Democrats will try to ensure it does, Democrat Roger Wilson who was preparing last night to succeed Carnahan as governor, will have the duty of appointing a replacement.

Carnahan was one of the most important moderate Democratic politicians in the US. His success in wresting control from the Republicans in the 1992 elections, and then maintaining it, mirrored Bill Clinton's strategy on the national stage. He was due to have attended the debate.